1. Field of the Invention
The present invention relates to a small electronic apparatus comprising a plurality of keys, a key input means which inputs signals for a prescribed device to execute prescribed operation from the keys respectively, and a sound generating means which generates different sounds for respective keys in response to the key input signals from the key input means.
2. Description of the Prior Art
FIG. 1 is a perspective view of a portable telephone as an example of a small electronic apparatus in the prior art. In FIG. 1, numeral 1 designates a body of the portable telephone, numeral 2 designates a receiver installed on the front side of the body 1 in the figure and having a speaker enclosed therein, numeral 3 designates a key input member comprising a plurality of keys for inputting telephone numbers, numeral 4 designates a small sound generator installed on the upper side of the body 1 in the figure and constituted, for example, by a piezoelectric buzzer enclosed in the body 1, and numeral 5 designates an antenna. In addition to the speaker and the piezoelectric buzzer, a circuit of the telephone is enclosed in the body 1.
FIG. 2 shows a circuit for generating confirmation sounds when the keys are operated respectively in the telephone of the prior art. In FIG. 2, numeral 6 designates a control circuit which processes the key input signals from the key input member 3 and supplies control signals responding to the key input signals to a single tone generating circuit 7 and a DTMF (Dual Tone Multi-Frequency) signal generator circuit 8. The single tone generator circuit 7 generates confirmation sounds of the actuated keys in the same single tone sounds for each key in response to control signals from the control circuit 6, and when the control signal is received the single tone generating circuit 7 generates an output signal during prescribed periods only. The output signal from the single tone generating circuit 7 is amplified by an amplifier 9 and supplied to a piezoelectric buzzer 10 of the small sound generator 4. The buzzer 10 is driven, and the same single tone sound for each key is generated during the prescribed period only as the confirmation sound of the key operation.
The DTMF signal generating circuit 8 generates DTMF signals as a combination of signals of different frequencies responding to the control signals from the control circuit 6, and outputs the DTMF signals to a telephone circuit (not shown) such as a modulator installed on side of the antenna 5. The DTMF signal is modulated, and then transmitted from the antenna 5 to a radio device installed in a telephone exchange in a telephone network. Thus selective signals for a telephone in communication destination by the DTMF signal, or a service request signal to the telephone exchange is transmitted to the telephone exchange.
FIG. 3 shows the assignment of frequency signals of the DTMF signals corresponding to each key. For example, the DTMF signal corresponding to "8" is the combination of signal of 852 Hz and signal of 1366 Hz, and if the key of "8" is depressed, the circuit shown in FIG. 2 generates the signal of 852 Hz and the signal of 1366 Hz simultaneously.
On the other hand, the DTMF signal is amplified by an amplifier 11 and then outputted as DTMF sound from a speaker 12 of the receiver 2 so that the operator can confirm the transmission of selective signals or the like. In this case, tone of the DTMF sound is different for each key, and the distinction of the actuated keys is possible with its tone.
Since the small electronic apparatus in the prior art is constituted as above described, when key confirmation of a plurality of keys installed in the key input member 3 is performed with the above-mentioned single tone sound and the DTMF sound, the following problems exist respectively.
In the first case of the single tone sound, although the key confirmation sound of sufficient volume is generated by the small sound generator 4 using the piezoelectric buzzer 10, since the sound generating band of the piezoelectric buzzer 10 generally available is often deviated from the frequency band of the DTMF signals from the DTMF signal generating circuit 8, the piezoelectric buzzer 10 cannot be driven by the DTMF signal and a different tone for each key generated at large sound volume whereby the operator confirms the operation key. That is, the distinction of the actuated keys cannot be performed.
For example, FIG. 4 shows a characteristic diagram of input frequency - output sound pressure level of a piezoelectric buzzer manufactured by JAPAN PIEZO CO., LTD. In this example, portions indicating good characteristics are seen in the vicinity of 600 Hz and the vicinity of 4000 Hz, and there is the deviation from the frequency band of the DTMF signals.
On the other hand, in the second case of the DTMF sound, since the DTMF signal drives the speaker 12 of the receiver 2, the distinction of the actuated keys can be performed, but the sound volume is small and the sound cannot be heard when the receiver 2 is separated from the user's ears. In order to obtain a sufficient sound volume, the receiver 2 having the speaker 12 with a diameter of 10 cm or more is required, thereby size for a small electronic apparatus, for example, the portable telephone inevitably becomes of large scale and heavy weight.